Labour's hopes of a summer fightback powered by economic recovery and the NHS row are dashed by a new Guardian/ICM poll showing that the Conservatives have gained ground on key policy areas and are now the overwhelming public choice to form the next government.

Despite a month of policy attacks from Gordon Brown's summer stand-ins at No 10, and controversy over the Conservative commitment to a state-funded NHS, the opposition has extended its lead to 16 points.

Labour has lost the August battle on health, with more voters thinking the Conservatives would improve the NHS than think the party would make it worse. While 48% think healthcare would be better under a Tory government, only 41% agree with Labour warnings that it would be worse. Even 24% of current Labour voters think the Tories would improve the NHS.

The Tory lead on other policies, including education, is bigger.

In an immediate general election, 25% say they would vote Labour – the joint lowest score in Guardian/ICM polling history and the worst for Labour in the series since June last year. The figure has only been lower once, in an ICM poll carried out for another paper during Labour's spring leadership crisis.

Labour support has dropped two points since the July Guardian poll and one point since a more recent ICM survey.

The Conservatives are on 41%, unchanged since the July Guardian poll, although down two on the more recent survey.

The party has scored 40% or more in 10 of the 12 ICM polls this year. This month's poll is also the 10th time in the ICM series that Labour has scored below 30% – nine of those have come since March 2008.

The Liberal Democrats meanwhile are on 19%, down one since July and unchanged since the more recent ICM poll.

Support for other parties, boosted by the European elections, has not fallen to previous levels. At 14%, up two (including the Greens, BNP and Ukip at 3% each, and nationalist parties on 5%), it has eaten into major party support.

At a general election these figures could see Labour lose more than 150 seats, and give the Conservatives a landslide majority approaching 100.

Asked, regardless of individual party preferences, whether a Tory government under David Cameron, or a Labour one under Gordon Brown, would be best for Britain, most people back the opposition.

While 58% of all voters – including 37% of people who voted Labour in 2005 – now think Cameron would be best, only 31% back Brown. The Conservatives lead Labour among all social classes and in all regions, although they are strongest among richer voters and those in the south.

Crucially, a Cameron government is the clear preference of most Liberal Democrats – 56% would rather see the Tories in power, against 36% who want Labour. In recent general elections, Lib Dem voters tended to gang up with Labour ones to stop the Conservatives winning marginal seats. At the next election, Labour could find itself the victim of Lib Dem tactical voting instead.

Meanwhile the poll suggests Labour would find little relief by choosing a new leader. Among possible candidates, most make little difference. David Miliband is the only one who matches Brown head to head, while Harriet Harman is the alternative leader most likely to deter voters.

Offered a straight choice between Brown and Harman, 20% pick Brown and only 8% Harman. She is also less popular among women – 21% to 5% – than among men – 20% to 10%.

• ICM interviewed a random sample of 1,004 adults aged 18+ by telephone between 21 and 23 August 2009. Interviews were conducted across the country and the results have been weighted to the profile of all adults. Percentages may not add to 100 because of rounding. ICM is a member of the British Polling Council and abides by its rules.